In an internal combustion engine (engine) mounted to a vehicle, for example, unburned air-fuel mixture and a burned combustion gas may leak from an combustion chamber into a crankcase through a gap between a cylinder and a piston.
When the combustion gas mixes with engine oil in the crankcase, the engine oil is degraded. To prevent the degrade of this engine oil, the engine is equipped with a blow-by gas reducing device of Positive Crankcase Ventilation (PCV) system that ventilates the inside of the crankcase.
In the engine including the blow-by gas reducing device, after a blow-by gas in the crankcase is separated into vapor and liquid through an oil separator, the blow-by gas is recirculated to an air intake pipe through a PCV valve (flow control valve) and a PCV hose (blow-by gas passage). The PCV hose is coupled to the air intake pipe at one end portion.
In situations where, for example, an intake air temperature is low, such as during idling of the engine with a low outside temperature, moisture included in the blow-by gas may freeze near the coupling portion of the PCV hose at an inner wall surface of the air intake pipe. This frozen moisture may grow into a large ice clump.
If such ice clump peels off and is transported to the downstream side of the air intake pipe by an intake airflow, the ice clump may collide with the compressor impeller of the turbocharger disposed downstream side of the air intake pipe. Especially, it is not preferred that a large ice clump collide with a thin-walled vane formed at the compressor impeller.
In contrast to this, the applicant of this application, for example, has proposed a technique described in Patent Literature 1. In this Patent Literature 1, forming a plurality of depressed portions in sections on the inner wall surface of the air intake pipe prevents a large ice clump from appearing on the inner wall surface of the air intake pipe.